1. Field of the Invention
The present invention relates to a modified metal oxide that can be grafted onto a polymeric substrate. The present invention also relates to a modified polymeric composition grafted with a modified metal oxide. Additionally, the present invention relates to methods for making the above described compositions.
2. Description of the Prior Art
In the use of polymers, it is frequently necessary to mix the polymers with certain fillers to impart desired properties, depending upon the nature of the filler and the intended end use of the polymer. There are many polymers that, because of their lack of functional groups, do not strongly interact with untreated fillers. Accordingly, and as taught in the prior art, it is generally necessary to use expensive materials, e.g., organo silanes, to treat the filler surface to make it compatible with the unfunctionalized polymer.
One method that has been used to functionalize a filler so as to make it more interactive with polymers lacking functional groups is to add a fraction of a maleated polymer to the bulk polymer/filler composition. For example, in the case of polypropylene, maleated polypropylenes have been used to improve the interaction of the polymers with filler materials. Generally speaking, maleated polypropylene is achieved by mixing an unmodified polypropylene, maleic anhydride and an organic peroxide to form a homogeneous mixture, e.g., by using an extruder. The extrudent modified polypropylene, i.e., a maleated polypropylene, being formed from the reaction of the peroxide with maleic anhydride and the polymer at the elevated temperature in the extruder. In the reaction, the peroxide decomposes to produce free radicals that initiate a reaction between the maleic anhydride and the polymer chain. In this regard, the peroxide radicals are capable of abstracting hydrogen from the polymer backbone, such that the polymer is reactive toward the maleic anhydride, the net result being the formation of a polypropylene chain that has maleic groups grafted along the backbone of the polymers.
This maleated polypropylene is subsequently extruded, or otherwise mixed, with fillers such as fiberglass, carbon black, silica, etc. together with a larger fraction of unmodified or bulk polymer, e.g., polypropylene as the bulk material. Thus, the mixture will comprise primarily unmodified polymer, e.g., polypropylene, a smaller but effective amount of a maleated (modified) polypropylene and the filler. It should be noted that in this mixture there is generally enough moisture available to result in hydrolysis of the maleic anhydride groups to produce acid groups that in turn interact and bond with the filler surface. Accordingly, the maleated polypropylene, in this way, acts as a coupling agent to form a bond between the filler and the bulk or unmodified polymer, e.g., polypropylene.
The polymers that can be modified in the above manner to act as coupling agents between both polymers and fillers include not only polypropylene but virtually any polyolefin, e.g., polyethylene, as well as polymers of unsaturated monomers such as isoprene, butadiene, styrene-butadiene, etc.
The prior art described above suffers from several disadvantages. For one, only a fraction of the maleic groups are in contact with the filler surface. Additionally, the free peroxide, which is usually hydrophobic and is well dispersed in the polymer, can reduce the molecular weight of the polymer chain by chain scission during the extrusion step. The free peroxide can also cause cross-linking of the polymer chains by combination of two polymer radicals, thereby dramatically changing the physical characteristics of the polymer. Lastly, the above described process requires two polymers, i.e., the modified polymer and the unmodified polymer.